The therapies of deep-brain stimulation (DBS) and auditory neuron stimulation have gained significantly clinical popularity over the past decades. The former has significant applications in the treatment of a variety of brain-controlled disorders, including movement disorders, while the latter has applications in the treatment of hearing impairment.
Generally, such treatments involve identifying a corresponding physiological target to be stimulated, surgically drilling a burr hole in the patient's skull or temporal bone to create an access to the corresponding physiological target, placing an electronic device in the corresponding physiological target through the drilled burr hole, and then applying appropriate stimulation signals through the implanted electrode device to the physiological target.
The placement portion of the treatment, involving stereotactic neurosurgical methodology, is very critical, and has been the subject of much attention and research. In particular, finding the deep brain target and then permanently placing the electrode lead so that it efficiently stimulates such target is very important.
Stereotactic neurosurgery is a field of neurosurgery in which a probe is advanced through a burr hole to a target of interest by means of a mechanical device attached to the skull with aiming based on pre-operative images. The probe may be a biopsy needle or an implantable device, but it is geometrically rigid, so that its tip, or working end portion, can be brought to a target of interest specified on a pre-operative image, by means of a geometrical calculation. For the past decade, the field has been advancing from the imposition of large, classical metal frames, which encompass the entire head of a patient, to the attachment of small platforms placed only over an entry site to reduce patient discomfort, facilitate surgical access, allow multiple targeting during one surgery via multiple platforms, and reduce procedure time, while maintaining the same level of accuracy.
More specifically, image-guided surgical (IGS) technology allows surgeons to navigate based upon registration of pre-intervention images (e.g., CT or MRI scans) to intraoperative anatomy. In the last 15 years, IGS systems using real-time tracking of surgical instruments have been FDA-approved and CE-marked for endoscopic sinus surgery and neurosurgical intervention. While versatile in allowing free-hand navigation during surgery, the accuracy of such IGS systems depends upon the type and placement of fiducial markers used to register to the pre-intervention scans. Accuracy of systems range from 1 to 2 mm for those which utilize bone-implanted fiducial markers [1] to 2-5 mm for those which depend upon skin-affixed fiducial marker systems (e.g., adhesively affixed skin markers and laser scanning of skin surfaces) [2].
For clinical applications where only a single or finite number of targets are to be accessed, the use of a highly versatile, real-time tracking IGS system may not offer the best solution. For such applications—biopsy and/or placement of electrodes into precise intracranial locations—the traditional stereotactic frame provides better overall accuracy without the need for elaborate tracking systems. The stereotactic frame is rigidly attached to a patient during both imaging and surgical intervention using sharp pins that pierce the skull. It offers increased levels of accuracy because the frame provides both the fiducial system and the targeting system. To date the most successful fiducial component of the stereotactic frame is the N-frame of Brown's design [3]. Target locations are determined by triangulation relative to the N-frame. Accuracy for such traditional stereotactic frames approaches 1 mm or better [4-6]. However, a major drawback is the bulky nature of the frames which are extraordinarily uncomfortable for patients and often obstructive of surgical exposure in the operating room.
To overcome the drawbacks of traditional stereotactic frames, microstereotactic frames were introduced. One such frame is a patient-customized microstereotactic frame [7] that mounts on bone-implanted anchors, which serve also as fiducial markers for targeting purposes. Now commercially available, the “StarFix microTargeting Platform” (FHC Inc., Bowdoin, Me., USA), henceforth referred to as the Starfix, is FDA-approved for placement of deep brain stimulating (DBS) electrodes [8]. In practice, a patient has at least three bone-implanted anchors placed, following which a CT, and possibly an MRI, is obtained. These fiducial markers are small and subcutaneously placed, so the patient can leave the medical facility between imaging and surgical intervention and return to normal activities of daily living. In the patient's absence, the surgical target is identified, as a path from the surface of the skull to the target. Next, a microstereotactic frame that mounts on the anchors and achieves the desired trajectory is manufactured via rapid prototyping. Because rapid-prototyping technology requires expensive equipment and expertise to perform, the current paradigm employs a centralized manufacturing facility from which the customized frames are shipped. Shipping imparts a delay of at least 48 h from the time of anchor placement until the time of surgical intervention. This delay is a disadvantage relative to the traditional stereotactic frame, but holds out the benefit of decreased human error as no adjustments are necessary once the Starfix is mounted. A recent phantom study indicated that the Starfix as used for DBS surgeries, provides submillimetric accuracy [9].
Another microstereotactic frame FDA-approved for DBS surgeries is the “NexFrame” (Medtronic, Minneapolis, Minn., USA) [10]. Unlike the Starfix, which is custom built for each patient, the NexFrame is universally adaptable to patient anatomy through the use of a real-time tracking IGS system, which is necessary to localize fiducials and aim the device. While the NexFrame system can be used immediately after placement of markers and CT/MRI scanning, it requires the availability of an IGS system, which costs upwards of $100,000. Resultant accuracy is limited by the tracking error inherent to the IGS system and human error during alignment of the device. A recent phantom study indicated that the NexFrame provides accuracy on the order of just over one millimeter [11].
Therefore, a heretofore unaddressed need still exists in the art to address the aforementioned deficiencies and inadequacies.